Apparatus and method to store information in a holographic data storage medium

ABSTRACT

A method is disclosed to store information in a holographic data storage medium. The method provides a hologram comprising an alignment pattern, and disposes that hologram into a holographic data storage medium during manufacture.

FIELD OF THE INVENTION

This invention relates to an apparatus and method to store informationin a holographic data storage medium.

BACKGROUND OF THE INVENTION

In holographic information storage, an entire page of information isstored at once as an optical interference pattern within a thick,photosensitive optical material. This is done by intersecting twocoherent laser beams within the storage material. The first, called thedata beam, contains the information to be stored; the second, called thereference beam, is designed to be simple to reproduce, for example asimple collimated beam with a planar wavefront.

The resulting optical interference pattern causes chemical and/orphysical changes in the photosensitive medium: a replica of theinterference pattern is stored as a change in the absorption, refractiveindex, or thickness of the photosensitive medium. When the storedinterference pattern is illuminated with one of the two waves that wereused during recording, some of this incident light is diffracted by thestored interference pattern in such a fashion that the other wave isreconstructed. Illuminating the stored interference pattern with thereference wave reconstructs the data beam, and vice versa.

A large number of these interference patterns can be superimposed in thesame thick piece of media and can be accessed independently, as long asthey are distinguishable by the direction or the spacing of thepatterns. Such separation can be accomplished by changing the anglebetween the object and reference wave or by changing the laserwavelength. Any particular data page can then be read out independentlyby illuminating the stored patterns with the reference wave that wasused to store that page. Because of the thickness of the hologram, thisreference wave is diffracted by the interference patterns in such afashion that only the desired object beam is significantly reconstructedand imaged on an electronic camera. The theoretical limits for thestorage density of this technique are on the order of tens of terabitsper cubic centimeter.

SUMMARY OF THE INVENTION

The invention comprises a holographic drive apparatus, comprising ahousing, a laser light source disposed within the housing, a beamsplitter disposed within the housing, a reflective spatial lightmodulator disposed within the housing, and a drive servo mechanismdisposed within the housing. In certain embodiments, the holographicdrive apparatus further comprises an optical sensor array disposedwithin the housing, wherein that wherein said optical sensor arraycomprises a rotatable input screen, a rotation error servo, a rotatableshaft extending outwardly from the rotation error servo, wherein thedistal end of the rotatable shaft is attached to the rotatable inputscreen.

The invention further comprises a method to store informationholographically, using the holographic drive apparatus. The methodreceives information, mounts a holographic data storage medium in theholographic drive apparatus, rotates the holographic data storagemedium, encodes in the rotating holographic data storage medium a firsthologram comprising an alignment pattern, encodes in the rotatingholographic data storage medium one or more data holograms comprisingthe information, and encodes in the rotating holographic data storagemedium a second hologram comprising the alignment pattern.

The invention further comprises a method to read informationholographically, using the holographic drive apparatus. The methodmounts an encoded holographic data storage medium in the holographicdrive apparatus, rotates the holographic data storage medium at anangular rotation φ, rotating the input screen at the angular rotation φ,illuminates the rotated encoded holographic data storage medium with areference beam to produce a data beam, and projects the data beam ontothe rotating input screen.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from a reading of the followingdetailed description taken in conjunction with the drawings in whichlike reference designators are used to designate like elements, and inwhich:

FIG. 1A is perspective view of a holographic data storage medium;

FIG. 1B is a cross-sectional view of the holographic data storage mediumof FIG. 1A;

FIG. 2 is a perspective view of a one embodiment of a holographic datastorage system shown encoding information into the holographic datastorage medium of FIGS. 1A and 1B;

FIG. 3 is a perspective view of a second embodiment of a holographicdata storage system shown encoding information into the holographic datastorage medium of FIGS. 1A and 1B;

FIG. 4 is a block diagram showing the holographic data storage system ofFIG. 3;

FIG. 5 is a perspective view of a one embodiment of a holographic datastorage system shown decoding information encoded into the holographicdata storage medium of FIGS. 1A and 1B;

FIG. 6 is a perspective view of a second embodiment of a holographicdata storage system shown decoding information encoded into theholographic data storage medium of FIGS. 1A and 1B;

FIG. 7 is a block diagram showing one embodiment of Applicants'holographic data storage system;

FIG. 8A is a perspective view of a portion of one embodiment ofApplicants' optical detector;

FIG. 8B shows a worm gear portion of the optical detector of FIG. 8A;

FIG. 8C shows a perspective view of a second embodiment of Applicants'optical detector;

FIG. 9A is a block diagram showing Applicants' holographic driveapparatus being used to encode information in a holographic data storagemedium;

FIG. 9B is a block diagram showing the holographic drive apparatus ofFIG. 9A being used to decode information written to a holographic datastorage medium;

FIG. 10 is perspective view of the holographic data storage medium ofFIGS. 1A and 1B;

FIG. 11 is a chart illustrating certain properties of the holographicdata storage medium of FIGS. 1A and 1B;

FIG. 12 is block diagram showing one embodiment of Applicants' alignmentpattern;

FIG. 13 is a flow chart summarizing certain steps of Applicants' methodto dispose a hologram comprising an orientation pattern into theholographic data storage medium of FIGS. 1A and 1B;

FIG. 14 is a flow chart summarizing the steps of Applicants' method todecode information encoded into the holographic data storage medium ofFIGS. 1A and 1B; and

FIG. 15 is a flow chart summarizing certain addition steps ofApplicants' method to decode information encoded into the holographicdata storage medium of FIGS. 1A and 1B.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention is described in preferred embodiments in the followingdescription with reference to the Figures, in which like numbersrepresent the same or similar elements. Reference throughout thisspecification to “one embodiment,” “an embodiment,” or similar languagemeans that a particular feature, structure, or characteristic describedin connection with the embodiment is included in at least one embodimentof the present invention. Thus, appearances of the phrases “in oneembodiment,” “in an embodiment,” and similar language throughout thisspecification may, but do not necessarily, all refer to the sameembodiment.

The described features, structures, or characteristics of the inventionmay be combined in any suitable manner in one or more embodiments. Inthe following description, numerous specific details are recited toprovide a thorough understanding of embodiments of the invention. Oneskilled in the relevant art will recognize, however, that the inventionmay be practiced without one or more of the specific details, or withother methods, components, materials, and so forth. In other instances,well-known structures, materials, or operations are not shown ordescribed in detail to avoid obscuring aspects of the invention.

Referring now to FIG. 2, holographic data storage system 200 compriseslaser light source 205, a beam splitter 210, transmissive Spatial LightModulator (“SLM”) 215, and mirror 280. In certain embodiments, laser 205emits blue light at a wavelength of about 405 nm. In certainembodiments, laser 205 emits or red light at a wavelength of about 650nm. In certain embodiments, laser 205 emits or infrared light at awavelength of about 780 nm. In certain embodiments, laser 205 emitsother wavelength(s) of light tuned to the recording and/or readingcharacteristics of holographic data storage medium 100 (FIGS. 1A, 1B).

In certain embodiments, transmissive SLM 215 comprises an LCD-typedevice. Information is represented by either a light or a dark pixel onthe SLM 215 display. The SLM 215 is typically translucent.

Laser light originating from the laser source 205 is split by the beamsplitter 210 into two beams, a carrier beam 220 and a reference beam230. The carrier beam 220 picks up the image 240 displayed by the SLM215 as the light passes through the SLM 215 to form data beam 260.Reference beam 230 is reflected off of first-surface mirror 280, to formreflected reference beam 290. Reflected reference beam 290 interfereswith the data beam 260 to form data hologram 130. Data hologram 130 isencoded into holographic storage medium 100 as interference pattern 270.

Referring now to FIGS. 3 and 4, holographic data storage system 300comprises laser light source 205, beam splitter 210, reflective spatiallight modulator 310, and holographic storage medium 100. The lightgenerated by source 205 is split by beam splitter 210 into referencebeam 320, and carrier beam 330. Using Apparatus 300, reference beam 320is not reflected.

In the illustrated embodiment of FIG. 3, reflective spatial lightmodulator (“RSLM”) 310 displays data image 240. In certain embodiments,reflective spatial light modulator 310 comprises an assembly comprisinga plurality of micro mirrors. In other embodiments, reflective spatiallight modulator 310 comprises a liquid crystal on silicon (“LCOS”)display device. In contrast to nematic twisted liquid crystals used inLCDs, in which the crystals and electrodes are sandwiched betweenpolarized glass plates, LCOS devices have the liquid crystals coatedover the surface of a silicon chip. The electronic circuits that drivethe formation of the image are etched into the chip, which is coatedwith a reflective (aluminized) surface. The polarizers are located inthe light path both before and after the light bounces off the chip.LCOS devices are easier to manufacture than conventional LCD displays.LCOS devices have higher resolution because several million pixels canbe etched onto one chip. LCOS devices can be much smaller thanconventional LCD displays.

Carrier beam 330 picks up image 240 as the light is reflected offreflective spatial light modulator 310 to form reflected data beam 340comprising image 240. Unreflected reference beam 320 interferes withreflected data beam 340 to form data hologram 130. Interference pattern270 encodes data hologram 130, and is formed within holographic storagemedium 100 thereby causing the photo-active storage medium to createinterference pattern 270.

FIG. 5 illustrates holographic data storage system 200 decodinginterference pattern 270 stored in media 100. In the illustratedembodiment of FIG. 5, holographic data storage system 200 comprisesoptical sensor array 510. Optical sensor array 510 is disposed adistance away from holographic storage medium 100 sufficient todigitally capture the reconstructed data beam 550 projected upon it. Todecode interference pattern 270 (FIG. 2), reference beam 230 isreflected off of mirror 280, to form reflected reference beam 290, whichis then incident on the encoded holographic storage medium 100. As thereference beam 290 interferes with interference pattern 270, areconstructed data beam 550 is generated, wherein that reconstructeddata beam 550 comprises an image 540 resembling the original image 240.Optical sensor array 510 digitally captures the information comprisingimage 540 on input screen 520.

Referring now to FIGS. 8A, 8B, and 8C, optical sensor array 510 furthercomprises rotation-error-servo (“RES”) 840. As those skilled in the artwill appreciate, a servo comprises a device comprising an externalshaft, such as rotatable shaft 850. Referring now to FIG. 8B, in certainembodiments RES 840 comprises a rotatable worm wheel 842, and shaft 850comprises a spirally-threaded portion 852, wherein spiral-threadedportion 852 meshes with worm wheel 842.

Rotatable shaft 850 can be positioned to specific angular positions bysending RES 840 a pre-defined coded signal. As long as that coded signalexists on input line 860, RES 840 will maintain the associated angularposition of shaft 850. As the coded signal changes, the angular positionof the shaft 850 changes.

RES 840 is interconnected by rotatable shaft 850 to rear portion 504 ofinput screen 520. RES 840 can cause input screen 520 to rotate in afirst direction, i.e. clockwise, or to rotate in a second and oppositedirection, i.e. counter-clockwise, by causing rotatable shaft 850 torotate in the first direction or in the second direction, respectively.

In the illustrated embodiment of FIG. 8A, optical sensor array 510further comprises detector controller 810, wherein RES 840 is disposedwithin detector controller 810. In the illustrated embodiment of FIG.8A, detector controller 810 further comprises processor 820 and memory830. In certain embodiments, memory 830 comprises non-volatile memory,such as and without limitation, battery backed-up RAM; a magnetic diskin combination with the associated software, firmware, and hardware, toread information from, and write information to, that magnetic disk; anoptical disk in combination with the associated software, firmware, andhardware, to read information from, and write information to, thatoptical disk; an electronic storage medium; and the like. By “electronicstorage medium,” Applicants mean, for example, a device such as a PROM,EPROM, EEPROM, Flash PROM, compactflash, smartmedia, and the like.

In the illustrated embodiment of FIG. 8A, detector controller 810further comprises microcode 832, wherein microcode 832 is written tomemory 830. Processor 820 utilizes microcode 832 to operate opticalsensor array 510. In the illustrated embodiment of FIG. 8C, signal inputline 860 interconnects RES 840 with an external controller, such asstorage controller 760 (FIG. 7). In certain embodiments, optical sensorarray 510 further comprises a floor stand 880 and vertical pillar 870.Pillar 870 may be at any angle other than vertical.

FIG. 6 shows holographic data storage system 300 being used to decodeinterference pattern 270. In the illustrated embodiment of FIG. 6,reference beam 608 is directed toward holographic storage medium 100such that reference beam 608 is diffracted by the interference pattern270 to form reconstructed data beam 650 comprising image 640 whichresembles the original image 240. Data beam 650 is projected onto inputscreen 520 of optical sensor array 510. Optical sensor array 510 thendigitally captures the information comprising image 640.

Referring now to FIGS. 9A and 9B, in certain embodiments laser lightsource 205, beam splitter 210, reflective spatial light modulator 310,and the rotatable input screen 520 of optical sensor array 510, aredisposed within holographic drive apparatus 900. In the illustratedembodiment of FIG. 9A, holographic drive apparatus 900 further compriseshousing 910.

In certain embodiments, holographic data storage medium 100 can beremoveably disposed within housing 910. In the illustrated embodiment ofFIG. 9A, holographic data storage medium 100 is releaseably attached toa drive servo mechanism comprising drive servo 940 and rotatable shaft950. Drive servo rotates rotatable shaft 950 thereby causing holographicdata storage medium 100 to rotate also.

In the illustrated embodiment of FIG. 9A, holographic drive apparatus900 further comprises controller 910. Controller 910 comprises processor920, memory 930, and microcode 935 written to memory 930. Memory 930 mayalso contain instructions 824. In certain embodiments, instructions 824comprise, inter alia, equations [1-4]. Controller 910 is interconnectedwith drive servo 940 via communication link 960, and with RES 840 viacommunication link 860. Controller 910, using processor 920 andmicrocode 935, can cause holographic data storage medium 100 to rotateat a first rotation rate, and can simultaneously cause input screen 520to rotate at a second rotation rate, wherein the first rotation rate mayequal the second rotation rate, and wherein the first rotation rate maydiffer from the second rotation rate.

In certain embodiments, memory 930 comprises non-volatile memory, suchas and without limitation, battery backed-up RAM; a magnetic disk incombination with the associated software, firmware, and hardware, toread information from, and write information to, that magnetic disk; anoptical disk in combination with the associated software, firmware, andhardware, to read information from, and write information to, thatoptical disk; an electronic storage medium; and the like. By “electronicstorage medium,” Applicants mean, for example, a device such as a PROM,EPROM, EEPROM, Flash PROM, compactflash, smartmedia, and the like.

FIG. 9A shows holographic drive apparatus 900 being used to encode datahologram 130 into holographic data storage medium 100. FIG. 9B showsholographic drive apparatus 900 being used to decode data hologram 130.In the illustrated embodiment of FIGS. 9A and 9B, data input screen 520outputs information using communication link 905. In certainembodiments, communication link 905 is interconnected with one or morehost computers. In certain embodiments, communication link 905 isinterconnected with a storage controller, such as for example storagecontroller 760 (FIG. 7).

FIG. 7 illustrates one embodiment of Applicants' data storage andretrieval system 700. In the illustrated embodiment of FIG. 7, datastorage and retrieval system 700 communicates with computing devices710, 720, and 730. In the illustrated embodiment of FIG. 7, computingdevices 710, 720, and 730 communicate with storage controller 760through a data communication fabric 740. In certain embodiments, fabric740 comprises one or more data switches 750. Further in the illustratedembodiment of FIG. 7, storage controller 760 communicates with one ormore holographic data storage systems. In the illustrated embodiment ofFIG. 7, data storage and retrieval system 700 comprises holographic datastorage systems 200 and 300, and holographic drive apparatus 900.

In certain embodiments, computing devices 710, 720, and 730, areselected from the group consisting of an application server, a webserver, a work station, a host computer, or other like device from whichinformation is likely to originate. In certain embodiments, one or moreof computing devices 710, 720, and/or 730 are interconnected with fabric740 using Small Computer Systems Interface (“SCSI”) protocol runningover a Fibre Channel (“FC”) physical layer. In other embodiments, theconnections between computing devices 710, 720, and 730, comprise otherprotocols, such as Infiniband, Ethernet, or Internet SCSI (“iSCSI”). Incertain embodiments, switches 750 are configured to route traffic fromthe computing devices 710, 720, and/or 730, directly to the storagecontroller 760.

In the illustrated embodiment of FIG. 7, storage controller 760comprises a data controller 762, memory 763, microcode 822, instructions824, processor 764, and data caches 766, 767, and 768, wherein thesecomponents communicate through a data bus 765. In certain embodiments,memory 763 comprises a magnetic information storage medium, an opticalinformation storage medium, an electronic information storage medium,and the like. By “electronic storage media,” Applicants mean, forexample, a device such as a PROM, EPROM, EEPROM, Flash PROM,compactflash, smartmedia, and the like.

In certain embodiments, the storage controller 760 is configured to readdata signals from and write data signals to a serial data bus on one ormore of the computing devices 710, 720, and/or 730. Alternatively, inother embodiments the storage controller 760 is configured to read datasignals from and write data signals to one or more of the computingdevices 710, 720, and/or 730, through the data bus 765 and the fabric740.

In certain embodiments, storage controller 760 converts a serial datastream into a convolution encoded data images. Those data images aretransferred to an SLM 215 or an RSLM 310.

In certain embodiments, the interconnected holographic data storagesystems 200, and 300, are located in different geographical places. Incertain embodiments, storage controller 760 distributes informationbetween two or more holographic data storage systems in order to protectthe information.

FIGS. 1A and 1B show holographic data storage medium 100 with geometriccenter-of-disk 105. In the illustrated embodiment of FIG. 1B,holographic data storage medium 100 comprisesFactory-written-calibration-hologram 120 (FIG. 1B),Drive-written-calibration-hologram 110 (FIG. 1B),Computer-Generated-Hologram 140 (FIG. 1B), and Data hologram 130 (FIG.1B), disposed along data plane 150. Data plane 150 is sandwiched betweensubstrate 104 and cover 102.

Factory-written-calibration-hologram 120 (FIG. 1B) andComputer-Generated-Hologram 140 (FIG. 1B) are disposed within theholographic data storage medium by the media manufacturer at the time ofmanufacture. By “at the time of manufacture,” Applicants mean prior tooffering the holographic data storage medium for sale, and beforeencoding any information, such as for example customer data, therein.

In certain embodiments, Computer-Generated-Hologram 140 (FIG. 1B) isstored on a read-only piece of media which is then physically implantedin the data plane 150 during a separate manufacturing process. In otherembodiments, Computer-Generated-Hologram 140 is stamped or lithographedonto holographic data storage medium 120 along data plane 150.

Factory-written-calibration-hologram 120 (FIG. 1B) is encoded directlyinto holographic data storage medium 100 by at the time of manufacture.Factory-written-calibration-hologram 120 (FIG. 1B) and/or the computergenerated-hologram 140 (FIG. 1B) are based on ranges of opticaltolerances. For the encoding holographic drive apparatus, such opticaltolerances include the refractive indices of all focusing lenses,refractive index of spatial light modulator (if a transmissive SLM isused), and refractive index of the beam splitter. For the media, theseoptical tolerances include the thicknesses and refractive indices ofeach layer of the holographic data storage medium.

Data hologram 130 (FIG. 1B) is encoded into the holographic data storagemedium after purchase. The apparatus used to encode a data hologram 130may not comprise the same apparatus later used to decode that datahologram 130 (FIG. 1B). Using a first apparatus to encode a datahologram, and a second apparatus to decode that hologram is calledinterchange. In certain embodiments, one or moredrive-written-calibration-holograms 110 are encoded along with one ormore data holograms 130 (FIG. 1B). Those one or moredrive-written-calibration-holograms 110 are used to ascertain theorientation of the data holograms 130 (FIG. 1B).

FIG. 10 shows a top view of holographic storage media 100. Holographicstorage medium 100 comprises a geometric center-of-disk 105. During awrite process, holographic data storage medium 100 is rotated about thewrite center-of-rotation 1010. During a read process, holographic datastorage medium is rotated about the read center-of-rotation 1020.

Ideally, geometric center of disk 105, write center-of-rotation 1010,and read center-of-rotation 1020 coincide, as with magnetic hard diskdrives. Because holographic data storage medium 100 can be removablymounted in different holographic drive apparatus 900, the geometriccenter-of-disk 105, write center-of-rotation 1010, and readcenter-of-rotation 1020 may differ. If information written toholographic storage media 120 is immediately read back, then writecenter-of-rotation 1010 and read center-of-rotation 1020 do coincide. Asa general matter, however, the geometric center-of-disk 105, writecenter-of-rotation 1010, and read center-of-rotation 1020 will notcoincide.

The distance between geometric center-of-disk 105 and writecenter-of-rotation 1010 comprises the write-eccentricity We 1030. Thedistance between the geometric center-of-disk 105 and the readcenter-of-rotation 1020 comprises the read-eccentricity Re 1040. Thedistance between the write center-of-rotation 1010 and the readcenter-of-rotation 1020 comprises the interchange-eccentricity Ie 1050.

The distance between write center-of-rotation 1010 and data hologram 130comprises the write-radius Rw 1060. The Rw 1060 vector defines thedirection Yw 1065 of the Y-axis of the data hologram. The direction ofthe X-axis of the data hologram, Xw 1085, is perpendicular to Yw 1065.The distance between read center-of-rotation 1020 and data hologram 130comprises the read-radius Rr 1070. The Rr 1070 vector defines thedirection Yr 1075 of the Y-axis of an image being read by an opticaldetector, such as optical sensor array 510. The direction of the X-axisof the detector, Xr 1080, is perpendicular to Yr 1075. Applicants'method rotates input screen 520 about the Z-axis during the read-processin order to align Xw 1085 and Yw 1065 with Xr 1080 and Yr 1075,respectively. The alignment is determined from the eccentricity duringthe write process and the eccentricity during the read process, thecombination of these eccentricities being the interchange eccentricityIe.

Preferably, the write center-of-rotation 1010 is located as close aspossible to the geometric center-of-disk 105 when writingFactory-Write-Calibrate-Hologram 120 (FIG. 1B) or storingComputer-Generated-Hologram 140 to minimize write-eccentricity We 1030for Factory-Write-Calibrate-Hologram 120 (FIG. 1B), orComputer-Generated-Hologram 140 (FIG. 1B). Minimizing thewrite-eccentricity also minimizes, the interchange eccentricity Ie whenreading the Factory-Precision-Write-Calibrate-Hologram 120 (FIG. 1B) orComputer-Generated-Hologram 140.

The Y-axis Yw 1065 and the X-axis Xw 1085 of the written hologram arerelated to the Y-axis Yr 1075 and the X-axis Xr 1080 via a matrixtransformation. Angle theta (θ) 1090 represents the angle ofinterchange-misalignment between the hologram as originally encodedusing a first holographic drive apparatus, and an optical detectordisposed in a second holographic drive apparatus decoding that hologram.Angle theta will vary between the write drive and the read drive, andwill also vary each time holographic data storage medium 100 is mountedto the same holographic drive apparatus. It is this angle theta 1090that is corrected by RES 840.

Because of the variation of angle theta, it is desirable to cache imagesand encode a plurality of data holograms at one time. Thus, once theinterchange angle theta is determined for anyone of those images, it isdetermined for all of those images written at the same time, as afunction of phi (φ) which represents the angular or “spin” rotation ofholographic data storage medium 100 about the Z-axis by the holographicdrive apparatus, FIGS. 1A, 1B, 10.

The angle of interchange-misalignment θ 1090 is related to theinterchange eccentricity Ie by Equation (1).Ie ² =Rw ² +Rr ²−2(Rw)(Rr) cos (θ)  (1)Solving for angle θ, and assuming that Ie is small, meaning thatRw=Rr=R, Equation (2) is derived:Cos(θ)=[2*R ² −Ie ²]/2*R ²=1−(Ie/R)²/2  (2)Solving for the sine of angle θ, Equation (3) is derived.Sin(θ)=(Ie/R)*[1−(Ie/R)²/4]^(1/2)  (3)Using Equations (2) and (3), the following matrix transformation,Equation (4), is constructed with write vector (Xw,Yw), read vector(Xr,Yr), and rotation matrix [M].

$\begin{matrix}{{\begin{matrix}{Xr} \\{Yr}\end{matrix}} = {{\begin{matrix}{M\left( {1,1} \right)} & {M\left( {1,2} \right)} \\{M\left( {2,1} \right)} & {M\left( {2,2} \right)}\end{matrix}}*{\begin{matrix}{Xw} \\{Yw}\end{matrix}}}} & (4)\end{matrix}$where the elements of matrix [M] are:M(1,1)=M(2,2)=1−(Ie/R)²/2M(1,2)=−M(2,1)=(Ie/R)[1−(Ie/R)²/4]^(1/2)Ie=(e)sin(φ), where angle phi (φ) represents the angular or “spin”rotation of holographic data storage medium 100 about the Z-axis by theholographic drive apparatus, FIGS. 1A, 1B, 10.

Thus, the elements of matrix [M] vary periodically with the rotation ofthe disk in the drive, and the detector input screen 520 will rotatecounterclockwise and clockwise about the Z-axis to account for angle φof the spin of disk 100 in addition to detector input screen 520rotating about the Z-axis to correct for angle θ. Angle φ is zero whenwrite center-of-rotation 1010 and read center-of-rotation 1020 arealigned. The determinant of matrix [M] equals one, as desired, so thatno image distortion is created by matrix [M].

Preferably, matrix [M] is close to the identity matrix as possible whenwriting Factory-Write-Calibrate-Hologram 120 (FIG. 1B) or storingComputer-Generated-Hologram 140 (FIG. 1B). One step in Applicants'method comprises centering holographic data storage medium 100 about thegeometric center of-disk 105 when writingFactory-Write-Calibrate-Hologram 120 (FIG. 1B) or storingComputer-Generated-Hologram 140 (FIG. 1B) at the time of manufacture.

For ease of input screen 520 tracking the encoded holograms, matrix [M]preferably varies only by angle φ for an entire circular track of data.In certain embodiments, Applicants' method encodes an entire track ofdata holograms at a time, in order that input screen 520 not erraticallyand unpredictably jump from one orientation to another about the Z-axis,when reading data holograms 130 (FIG. 1B). That data track may compriseeither a circular or a spiral arc.

When decoding reconstructed data beams, optical sensor array 510utilizes angle θ, wherein angle θ varies with the rotation of the angleφ as well as the write center-of-rotation 1010 and the readcenter-of-rotation 1020. One or more Drive-Written-Calibrate-Holograms110 (FIG. 1B) are preferably written by the drive when each data trackis written, to aid in the alignment of input screen 520 with the datahologram 130 (FIG. 1B) to be read.

In certain embodiments, Applicants' method mounts a holographic datastorage medium, such as holographic data storage medium 100 in aholographic drive apparatus, such as holographic drive apparatus 900.The method then rotates the holographic data storage medium, encodes inthe rotating holographic data storage medium a first drive-writtenhologram comprising an alignment pattern, encodes in the rotatingholographic data storage medium one or more data holograms comprisinginformation, and encodes in the rotating holographic data storage mediuma second drive-written hologram comprising the alignment pattern,wherein the first drive-written hologram, the one or more dataholograms, and the second drive-written hologram define a circulartrack.

FIG. 13 summarizes the steps of Applicants' method to dispose a Computergenerated hologram 140 (FIG. 1B) comprising an alignment pattern, or aFactory-Written Calibrate Hologram 120 (FIG. 1B) comprising an alignmentpattern, in a holographic data storage medium at the time ofmanufacture. In step 1310, Applicants' method determines the statisticalmidpoint and ranges of all optical tolerances relating to theholographic data storage medium. In certain embodiments, the opticaltolerances of step 1310 include one or more of the thicknesses andrefractive indices of each layer of the holographic data storage medium.Referring now to FIG. 11, an example optical tolerances is shown viagraph 1100 of medium thicknesses versus refractive indices of each layerof media 100. In FIG. 11, layer 0 may represent cover 102 and layer 1may represent substrate 104, for transmissive holographic media 100.

Referring once again to FIG. 13, in step 1320 Applicants' methoddetermines whether to dispose one or more computer generated holograms(GCH) 140 (FIG. 1B) in the holographic data storage medium. IfApplicants' method elects in step 1320 to dispose a computer generatedhologram comprising an orientation pattern in the holographic datastorage medium, then the method transitions from step 1320 to step 1330wherein the method provides a master of a computer generated hologram140 comprising orientation pattern 1200 (FIG. 12) at the statisticalmidpoint and statistical “corners” (extremes) of the optical path.

Referring now to FIG. 12, alignment pattern 1200 includes alignment area1220 comprising the outer periphery and contains alignment marks 1230,1240, 1250, and 1260, for the determination of angle theta by opticalsensor array 510 for use by rotation error servo RES 840 (FIGS. 8A, 8C,9A, 9B). The alignment marks 1230, 1240, 1250, and 1260, are disposed onthe outer periphery to give the maximum precision to the determinationof angle theta 1090. Data 1210 is surrounded by alignment area 1220.Data 1210 may comprise Factory-written-calibration-hologram 120 (FIG.1B), Drive-written-calibration-hologram 110 (FIG. 1B),Computer-Generated-Hologram 140 (FIG. 1B), and Data hologram 130 (FIG.1B).

In certain embodiments, the master of a computer generated hologram)comprises a bit stream suitable for use in a laser writer, such assimilar to a DVD-ROM master writer, for producing a stamped or writtencalibration hologram. In certain embodiments, the master comprises atwo-dimensional interference-pattern for use in a photolithographic orlithographic-immersion stepper tool to produce a specific pattern. Incertain embodiments, the master comprises a three-dimensionalinterference pattern for use in a holographic imaging writer.

In step 1340, Applicants' method disposes the computer-generatedhologram comprising an alignment pattern 1200 (FIG. 12) in theholographic data storage medium at the time of manufacture. In certainembodiments, step 1340 comprises stamping, embedding, or lithographingthe computer generated hologram comprising an alignment pattern, such asfor example CGH 140 (FIG. 1B) in the holographic data storage medium,such as holographic data storage medium 100. In certain embodiments,that CGH 140 is disposed along data plane 150 (FIG. 1B).

If Applicants' method elects in step 1320 not to dispose a computergenerated hologram 140 (FIG. 1B) in the holographic data storage medium,then the method transitions from step 1320 to step 1350 wherein themethod provides a holographic drive apparatus, such as holographic driveapparatus 900 (FIGS. 9A, 9B), and wherein a ray trace is made of thatholographic drive apparatus to determine the “footprint” of its opticalproperties, and wherein the optical components of the drive are selectedwith nominal (midpoint) refractive indices, per drive specifications. Incertain embodiments, the optical properties of step 1350 comprise therefractive indices of all focus lenses, refractive index of spatiallight modulator if a transmissive SLM is used, and the refractive indexof the beam splitter.

In step 1360, Applicants' method mounts the holographic data storagemedium 100 is mounted on the holographic drive apparatus of step 1350.In step 1370, the holographic data storage medium is centered in theholographic drive apparatus to minimize write eccentricity We 1030, andhence reduce interchange eccentricity Ie 1050 when subsequently readingthe Factory-Precision-Written-Calibrate-Holograms 120 (FIG. 1B) encodedin the holographic data storage medium in step 1380, to include analignment pattern 1200 (FIG. 12). In certain embodiments, step 1370utilizes the method described in U.S. Patent Application 2004/0061966A1,which is hereby incorporated by reference. Applicants' method reducesthe magnitude of eccentricity e, and thus Ie, in Equation (4) by afactor of two if the statistical distribution of read center-of-rotation1020 is uniform. In step 1380, Applicants' method encodes aFactory-written-calibration-hologram 120 (FIG. 1B) into the holographicdata storage medium 100.

Applicants' invention includes a method to read images encoded into aholographic data storage medium, such as holographic data storage medium100. Referring now to FIG. 14, in step 1410 Applicants' method providesa holographic drive apparatus, such as holographic drive apparatus 900(FIGS. 9A, 9B).

In step 1420, Applicants' method mounts a holographic data storagemedium comprising one or moreFactory-Precision-Written-Calibrate-Holograms 120 (FIG. 1B), and/or oneor more computer-generated holograms 140 (FIG. 1B), and/or one or moreDrive-written-calibration-holograms 110 (FIG. 1B), and furthercomprising information encoded as one or more data holograms 130. Eachof these holograms 120, 140, 110, and 130 further comprise an alignmentpattern 1200 (FIG. 12).

In step 1430, Applicants' method rotates the mounted holographic datastorage medium of step 1420 at an angular rotation φ. In step 1440,Applicants' method rotates the rotatable optical detector, such asoptical sensor array 510, at an angular rotation φ.

In step 1450, Applicants' method illuminates the rotating holographicdata storage medium with a reference beam to generate a plurality ofdata beams. In step 1460, Applicants' method projects the plurality ofdata beams generated in step 1450 onto the rotating optical sensor array510.

In certain embodiments, step 1440 is replaced by the steps illustratedin FIG. 15. Referring now to FIG. 15 in step 1510 Applicants' methodselects a data hologram, such as data hologram 130 (FIG. 1B), encoded inthe holographic data storage medium of step 1420 (FIG. 14). In step1520, Applicants' method calculates an angle of interchange misalignmentθ 1090, such as by use of equations [1-4]. In step 1580, Applicants'method rotates the optical sensor array 510 of step 1410 based upon boththe angular rotation φ and the angle of interchange misalignment θ 1090.

In certain embodiments, step 1520 comprises steps 1530 through 1570. Instep 1530, Applicants' method determines a read center of rotation forthe selected data hologram. In certain embodiments, step 1530 furthercomprises reading one or more encodedFactory-Precision-Written-Calibrate-Holograms 120 (FIG. 1B), and/or oneor more computer generated holograms 140 (FIG. 1B), and using the one ormore alignment patterns comprising those one or moreFactory-Precision-Written-Calibrate-Holograms 120 (FIG. 1B), and/or oneor more computer generated holograms 140 (FIG. 1B), to determine thatread center of rotation. In step 1540, Applicants' method determines aRr radius comprising the distance between the read center of gravity andthe location of the selected data hologram.

In step 1550, Applicants' method determines a write center of rotationfor the selected data hologram. In certain embodiments, step 1550further comprises reading one or more encodedDrive-Written-Calibrate-Holograms 110 (FIG. 1B), and using the one ormore alignment patterns comprising those one or moreDrive-Written-Calibrate-Holograms 110 (FIG. 1B) to determine the writecenter of rotation. In step 1560, Applicants' method determines a Rwradius comprising the distance between the write center of gravity andthe location of the selected data hologram.

In step 1570, Applicants' method calculates an interchange-eccentricityIe comprising the distance between the read center of gravity of step1530 and the write center of gravity of step 1550. Applicants' methodutilizes the write center of gravity of step 1530, the Rr radius of step1540, the read center of gravity of step 1550, the Rw radius of step1560, and the interchange-eccentricity Ie of step 1570 to calculate theangle of interchange misalignment θ 1090 in step 1520 via equations[1-4].

Applicants' method as summarized in FIG. 14, optionally using the stepssummarized in FIG. 15, may be used by a storage services provider toprovide data storage services to one or more data storage servicescustomers.

In certain embodiments, individual steps recited in FIGS. 13, 14, and/or15, may be combined, eliminated, or reordered.

In certain embodiments, Applicants' invention includes instructions,such as instructions 824 (FIGS. 7, 8A, 9A), residing in memory 763 (FIG.7), and/or memory 830 (FIG. 8A), and/or memory 930 (FIG. 9A), wherethose instructions are executed by a processor, such as processor 764(FIG. 7), and or processor 820 (FIG. 8A), and/or processor 920 (FIG.9A), to perform one or more of steps 1360, 1370, and/or 1380, recited inFIG. 13, and/or one or more to steps 1420, 1430, 1440, and/or 1450,recited in FIG. 14, and/or one or more of steps 1510, 1520, 1530, 1540,1550, 1560, 1570, and/or 1580, recited in FIG. 15.

In certain embodiments, Applicants' invention includes instructionsresiding in any other computer program product, where those instructionsare executed by a computer external to, or internal to, holographic datastorage system 200, holographic data storage system 300, holographicdata storage and retrieval system 700, and/or holographic driveapparatus 900, to perform one or more of steps 1360, 1370, and/or 1380,recited in FIG. 13, and/or one or more to steps 1420, 1430, 1440, and/or1450, recited in FIG. 14, and/or one or more of steps 1510, 1520, 1530,1540, 1550, 1560, 1570, and/or 1580, recited in FIG. 15. In either case,the instructions may be encoded in an information storage mediumcomprising, for example, a magnetic information storage medium, anoptical information storage medium, an electronic information storagemedium, and the like. By “electronic storage media,” Applicants mean,for example, a device such as a PROM, EPROM, EEPROM, Flash PROM,compactflash, smartmedia, and the like.

While the preferred embodiments of the present invention have beenillustrated in detail, it should be apparent that modifications andadaptations to those embodiments may occur to one skilled in the artwithout departing from the scope of the present invention as set forthin the following claims.

1. A holographic drive apparatus comprising a laser light source, a beamsplitter, a drive servo mechanism, a processor, a rotatable opticaldetector, and computer readable program code disposed in a computerreadable medium, said computer readable program code being useable withsaid processor to encode information into a holographic data storagemedium, the computer readable program code comprising a series ofcomputer readable program steps to effect: rotating a holographic datastorage medium releaseably mounted to said drive servo, wherein saidholographic data storage medium comprises an encoded computer generatedhologram comprising an alignment pattern; retrieving a stored imagecomprising said alignment pattern; receiving information; rotating saidholographic data storage medium; encoding in said rotating holographicdata storage medium a first drive-written hologram comprising saidalignment pattern; encoding in said rotating holographic data storagemedium one or more data holograms comprising said information; encodingin said rotating holographic data storage medium a second drive-writtenhologram comprising said alignment pattern; wherein said firstdrive-written hologram, said one or more data holograms, and said seconddrive-written hologram define a circular track rotating said holographicdata storage medium at an angular rotation φ; providing a referencebeam; illuminating the rotated holographic data storage medium with saidreference beam to produce a data beam; rotating said optical detector atsaid angular rotation φ; projecting said data beam onto said rotatingoptical detector.
 2. The holographic drive apparatus of claim 1, saidcomputer readable program code further comprising a series of computerreadable program steps to effect: selecting a data hologram encoded insaid holographic data storage medium; calculating an angle ofinterchange misalignment θ for said data hologram; and rotating saidoptical detector based upon both said angular rotation φ and said angleof interchange misalignment θ.
 3. The holographic drive apparatus ofclaim 2, wherein said computer readable program code to calculate anangle of interchange misalignment step further comprises a series ofcomputer readable program steps to effect: determining a read center ofrotation for said data hologram; determining an Rr radius comprising thedistance between said read center-of-rotation and said data hologram;determining a read center of rotation for said data hologram;determining an Rw radius comprising the distance between said writecenter-of-rotation and said data hologram; calculating aninterchange-eccentricity Ie comprising the distance between said readcenter of rotation and said write center of rotation; wherein saidcalculating an angle of interchange misalignment step further comprisescalculating an angle of interchange misalignment θ using said Rr radius,said Rw radius, and said interchange-eccentricity Ie.
 4. A computerprogram product encoded in a computer readable medium disposed in aholographic drive apparatus comprising a laser light source, a beamsplitter, a drive servo mechanism, a spatial light modulator, arotatable optical detector, and a holographic data storage mediumremoveably disposed in said holographic drive apparatus, wherein saidholographic data storage medium comprises at least one factory-writtenhologram comprising an alignment pattern, comprising: computer readableprogram code which causes said programmable computer processor toretrieve a stored image comprising said alignment pattern; computerreadable program code which causes said programmable computer processorto encode in said holographic data storage medium a drive-writtenhologram into said holographic data storage medium, wherein saiddrive-written hologram comprises said alignment pattern; computerreadable program code which causes said programmable computer processorto receive information; computer readable program code which causes saidprogrammable computer processor to rotate said holographic data storagemedium; computer readable program code which causes said programmablecomputer processor to encode in said rotating holographic data storagemedium a first drive-written hologram comprising said alignment pattern;computer readable program code which causes said programmable computerprocessor to encode in said rotating holographic data storage medium oneor more data holograms comprising said information; computer readableprogram code which causes said programmable computer processor to encodein said rotating holographic data storage medium a second drive-writtenhologram comprising said alignment pattern; wherein said firstdrive-written hologram, said one or more data holograms, and said seconddrive-written hologram define a circular track; computer readableprogram code which causes said programmable computer processor to rotatesaid holographic data storage medium at an angular rotation φ; computerreadable program code which causes said programmable computer processorto provide a reference beam; computer readable program code which causessaid programmable computer processor to illuminate the rotatedholographic data storage medium with said reference beam to produce aplurality of data beams; computer readable program code which causessaid programmable computer processor to rotate said optical detector atsaid angular rotation φ.
 5. The computer program product of claim 4,further comprising: computer readable program code which causes saidprogrammable computer processor to select a data hologram encoded insaid holographic data storage medium; computer readable program codewhich causes said programmable computer processor to calculate an angleof interchange misalignment θ for said data hologram; computer readableprogram code which causes said programmable computer processor to rotatesaid optical detector based upon both said angular rotation cp and saidangle of interchange misalignment θ.
 6. The computer program product ofclaim 5, further comprising: computer readable program code which causessaid programmable computer processor to determine a read center ofrotation for said data hologram; computer readable program code whichcauses said programmable computer processor to determine an Rr radiuscomprising the distance between said read center-of-rotation and saiddata hologram; computer readable program code which causes saidprogrammable computer processor to determine a read center of rotationfor said data hologram; computer readable program code which causes saidprogrammable computer processor to determine an Rw radius comprising thedistance between said write center-of-rotation and said data hologram;computer readable program code which causes said programmable computerprocessor to calculate an interchange-eccentricity Ie comprising thedistance between said read center of rotation and said write center ofrotation; computer readable program code which causes said programmablecomputer processor to calculate an angle of interchange misalignment θusing said Rr radius, said Rw radius, and said interchange-eccentricityIe.
 7. A method to provide data storage services by a data storageservices provider to one or more data storage services customers,comprising the steps of: receiving information from a data storageservices customer; providing a holographic drive apparatus comprising alight source, a drive servo mechanism, a spatial light modulator, arotatable optical detector, and an alignment pattern; mounting aholographic data storage medium in said holographic drive apparatus;rotating said holographic data storage medium; encoding in said rotatingholographic data storage medium a first hologram comprising saidalignment pattern; encoding in said rotating holographic data storagemedium one or more data holograms comprising said information; encodingin said rotating holographic data storage medium a second hologramcomprising said alignment pattern; wherein said first drive-writtenhologram, said one or more data holograms, and said second drive-writtenhologram define a circular track; rotating said holographic data storagemedium at an angular rotation φ; rotating said optical detector at saidangular rotation φ; illuminating the rotated encoded holographic datastorage medium with a reference beam to produce a data beam; projectingsaid data beam onto said rotating optical detector.
 8. The method ofclaim 7, further comprising the steps of: selecting a data hologramencoded in said holographic data storage medium; calculating an angle ofinterchange misalignment θ for said selected data hologram; rotatingsaid optical detector based upon both said angular rotation φ and saidangle of interchange misalignment θ.
 9. The method of claim 8, whereinsaid calculating an angle of interchange misalignment step furthercomprises the steps of: determining a read center of rotation for saiddata hologram; determining an Rr radius comprising the distance betweensaid read center-of-rotation and said data hologram; determining a readcenter of rotation for said data hologram; determining an Rw radiuscomprising the distance between said write center-of-rotation and saiddata hologram; calculating an interchange-eccentricity Ie comprising thedistance between said read center of rotation and said write center ofrotation; wherein said calculating an angle of interchange misalignmentstep further comprises calculating an angle of interchange misalignmentθ using said Rr radius, said Rw radius, and saidinterchange-eccentricity Ie.